This invention relates to slicing machines that are principally used for slicing food products, particularly slicing cheese, meat and pressed or moulded meat products.
Such a slicing machine comprises a rotating blade which either has a spiral cutting edge or has a circular cutting edge and is mounted for planetary motion, and means to feed the product towards the blade so that upon each revolution or each gyration of the blade, one slice is cut from the face of the product. The means to feed the product may be a continuous conveyor but usually the slicer includes a fixed platform on which the product is placed and a feeding head which engages the rear face of the product and which urges it towards the blade. The feeding head is moved by a hydraulic ram or by a leadscrew driven by a stepping or variable speed electric motor.
A slicing machine is usually required to produce groups of slices and each group is then packaged separately. This may be achieved by having the slicing machine discharge onto a constant speed conveyor and by interrupting the feed of the product towards the blade for a period of time, each time a predetermined number of slices have been cut from its face. However, more usually, a jump conveyor is located downstream from the blade of slicing machine. In this case the jump conveyor moves forward at a first speed whilst the slices that form each group are being cut and then, after the number of slices required for each group have been cut, the jump conveyor moves at a second speed which is considerably faster than the first speed, and then returns to the first speed for the slices to form the next group. In this way the slices are cut at a uniform rate from the product but the increase in speed of the jump conveyor after each group of slices has been cut, results in a series of groups of slices being formed on the jump conveyor.
It is desirable for each group of slices to have a predetermined, required, weight and various attempts and proposals have been made in the past for ways to achieve this. One way is for the product to be moved towards the blade at a constant speed so that the slicer always gives a particular required number of slices and these will be under the required weight, and then, upon subsequent weighing of each group of slices a portion of a single slice is added to the package by hand to make it up to the required weight. Firstly, this is very labour intensive and secondly it is undesirable from a commercial point of view because it is preferred that each pack contains only whole slices.
More recently, slicing machines have been made more sophisticated by the inclusion downstream of the slicing machine of means to weigh a group of slices cut by the slicing machine, and then, in dependence upon the weight of this preceding group, vary the speed of movement of the product towards the blade by a feedback system to ensure, as far as possible, that each slice has a particular, predetermined weight. This apparatus is very complicated and inevitably there is some time lag between the cutting of a group of slices and the determination that that group has been cut too thickly or too thinly, and then a further time lag before the feed of the product towards the blade is changed to make a correction. Most food products are natural in origin and therefore not uniform and accordingly it has been found that when the slice thickness is adjusted in this way it does not always produce the desired effect and may even increase the errors.
We have also proposed in our earlier patent specification GB-A No. 2099609 that some account can be taken of differences between pieces of meat or meat products by simply weighing the piece of meat or meat product and also measuring its length and then setting the feed rate of the product towards the blade to a uniform value in accordance with the average weight/unit length.
Whilst this technique produces surprisingly good results compared to the weight feedback systems, food products may not be of uniform density along their length. The density varies with such factors as the meat/fat ratio, with preferential liquid retention zones and surface dehydration and these factors naturally depend upon the source, the nature of the particular cut of the meat and the processes used in the pretreatment of the meat or other product including refrigeration cycles and any pressing that has taken place. In addition to these variations in density, variations also occur in the overall shape and hence cross-sectional area of some products particularly meat or meat product. Changes in the cross-sectional area naturally affect the weight of slices of a particular thickness of that are cut. In spite of these great differences that occur in such naturally produced materials we have discovered that, for example, products of a particular type such as sides of back bacon all have a roughly similar weight distribution along their length. Naturally the physical cross-sectional area of individual sides of bacon vary, as does their weight and overall length, but in all these cases, the weight distribution profile of sides of bacon have the same general form and for back bacon it has a form somewhat resembling a sinusoidal curve. For moulded meat products, such as those formed in a vertical tapered mould the typical weight distribution profile is a square law curve.